1. Technical Field
This invention relates to the general field of particle beam lithography and, more particularly, to real-time correction of the lithography process to compensate for proximity heating of the resist. The particles may be electrons, photons, ions or uncharged particles, but the discussion will refer particularly to electron beam lithography, which presently is the method most commonly used to manufacture lithographic masks.
2. Description of the Related Art
The production of precise patterns on surfaces is a necessary stage in the fabrication of integrated circuits, and finds applicability in many other commercial environments as well. The typical method for creating such patterns is to coat the surface to be patterned with a chemical that undergoes a chemical transformation upon exposure to energy, a xe2x80x98resistxe2x80x99. Positive resists undergo chemical transformation on exposure to energy leading to removal of resist from the surface in the regions so exposed. Negative resists undergo other chemical transformations, such as cross-linking, leading to removal of resist in regions not exposed to energy. After the resist in the appropriate pattern has been removed by exposure to the appropriate pattern of energy, the underlying surface may be subjected to further chemical etching or material deposition. Following surface etching or deposition, the remaining resist is removed.
The energy incident on the resist is typically either electromagnetic or a beam of particles, typically ions or electrons (xe2x80x98e-beamxe2x80x99). The energy may be directed onto the resist in one of two general ways: 1) through a mask having both transparent and opaque regions therein permitting selective passage of the incident energy to create the desired pattern of exposure on the underlying resist, or 2) as a focused beam, guided so as to impact selectively only those areas requiring exposure.
Exposure through a mask is the most common technique presently used for producing numerous identical patterns at reduced costs. However, the mask itself must first be made, most commonly by focused beam impact. Thus, focused beam exposure of resists is a necessary step in the production of masks for lithography.
Direct beam xe2x80x9cwritingxe2x80x9d of patterns, although generally much slower, has several advantages over printing from masks. Among these are avoiding the complications of alignment and registration of the mask, and more precise patterning accomplished by precisely focused beams.
The electronics industry is driving to reduce the feature size of components. Smaller components, which are desired because of their higher switching speeds and lower power consumption, place increasingly critical demands on pattern accuracy. Precise patterning requires precise exposure of resist: Sharp and accurately positioned boundaries are desired between exposed regions and unexposed regions for both types of resist, to permit the pattern designer to use more densely packed components without interference and overlap of imprecisely exposed adjacent pattern.
For concreteness of our description, we will consider the case of positive resists, which are removed from the underlying layer for subsequent etching or deposition where the positive resist is exposed to the incident e-beam. Completely analogous effects are present for negative resists as well understood in the art.
Precise exposure of resist requires a detailed understanding of the sensitivity of the resist to e-beam exposure. The exposure of resist to an e-beam, called the dose, is typically measured in microcoulombs per square centimeter (xcexcC/cm2). The xe2x80x9csensitivityxe2x80x9d of the resist means the electron dose (in xcexcC/cm2) necessary to create the desired pattern in the resist upon development. This sensitivity is a function of the resist composition, the energy of the incident electron beam, the temperature of the resist, the resist development process and other factors. The changes of resist sensitivity with its temperature at the time writing occurs are a particular concern.
Changing the temperature of the resist changes its sensitivity, which may require changing the dose of electrons in order to achieve proper exposure. Failing to take into account changes in resist sensitivity with temperature may lead to overexposure of the resist, exposure of the resist in regions not intended to be exposed, and less precise patterns. As will be described in greater detail below, pattern xe2x80x9cbloomingxe2x80x9d (illustrated more particularly in FIG. 2, described below) is the undesired result.
A particular concern is a phenomenon known as xe2x80x9cmedium range proximity heating.xe2x80x9d In high voltage lithography, most of the electron beam energy passes through the resist and the underlying mask layer (typically very thin) and penetrates the substrate where most of it is deposited. (An exception occurs when thin substrates are used, typically in the manufacture of X-ray masks, where the substrate is itself a film so thin that most of the beam energy passes through it). Electron diffusion in a thick substrate deposits the heat from a single e-beam pulse or xe2x80x98flashxe2x80x99 in the substrate typically in a volume much larger in lateral extent (perpendicular to the e-beam direction) and also in depth into the substrate than the flash size. Subsequent thermal conduction transports a portion of this heat to the substrate surface where it heats the resist in a zone that may be tens of microns in lateral extent some microseconds following the flash, increasing to a millimeter across after many milliseconds. (Exact numbers will depend on beam energy, the composition of the substrate and its thermal properties). Such proximity heating depends on the previously written pattern and the time history of the pattern writing. It may result in temperature changes of tens of degrees. This variability makes proximity heating particularly challenging to estimate in designing a process for high accuracy e-beam writing
There are two other proximity heating effects, termed xe2x80x9cnearby flash heatingxe2x80x9d and xe2x80x9cglobal heatingxe2x80x9d which are not to be confused with this xe2x80x9cmedium range substrate heating.xe2x80x9d These other effects may also require real time compensation but occur on very different time and distance scales and require different methods. Nearby flash heating can be compensated by simple table lookup of pre-computed temperatures. However, this lookup may have to occur very rapidly, e.g., on a nanosecond time scale. Global heating of the entire substrate may also require real-time prediction which cannot ignore the mask boundaries. It causes thermal expansion on a time scale of minutes and hours, even though the temperature rise is not large.
xe2x80x9cProximity heatingxe2x80x9d as used herein is not to be confused with xe2x80x9cproximity effectxe2x80x9d which is a term commonly used to describe the unwanted exposure of the resist by electrons backscattered from the substrate. Corrective measures are taken to compensate for this undesired exposure which, however, also is affected by heating of the resist.
FIGS. 1A and 1B illustrate beams of low and high energy electrons, respectively, incident on a substrate. The general mode of operation of e-beam lithography makes use of a focused beam of electrons, accelerated through a voltage, typically 1000 volts (1 keV) and above. Lower voltage e-beams are more effective at exposing the resist. Higher voltage e-beams are preferred for their ability to be formed into more precisely focused beams, and with less scattering in the resist layer, resulting in more accurate lithography and the ability to fabricate smaller patterns. xe2x80x9cHigh voltagexe2x80x9d e-beams herein is commonly understood to mean e-beam energies above approximately 10 keV. Beam energies as high as 50-100 keV are used. However, high energy e-beams produce undesired heating side effects.
FIG. 1A depicts schematically and in cross section a beam of low energy electrons (less than approximately 10 keV) 104a incident on a layer of resist 103a. Typically, resist layer 103a will be relatively thin, around 0.5 xcexcm (xe2x80x9cmicronsxe2x80x9d=10xe2x88x926 meter). Resist 103a overlies the layer to be etched 102a, all of which typically are supported by a reasonably thick substrate. For the manufacture of lithography masks, layer 102a will be generally be the mask material, typically a film of proprietary composition containing chromium and commonly very thin compared to the resist layer. Substrate 100a is typically glass and may be considered to be infinitely thick as none of the effects encountered in e-beam lithography relevant to the invention described herein are affected by the lower surface or edges of a thick glass layer (not depicted in FIG. 1A). Very thin substrates as would typically be encountered in the fabrication of x-ray lithography masks are an exception. FIG. 1A (in common with all other figures herein) is schematic only and not drawn to scale.
For a low energy beam as depicted in FIG. 1A, significant spreading of the e-beam occurs in the resist layer, commencing virtually immediately upon impact with the resist surface. The spreading of low energy e-beams in width may be commensurate with the depth of penetration. Thus, low energy e-beams tend to scatter in the resist layer, exposing thereby a larger range of resist than desired and exposing the resist in different patterns than intended, broader than the incident beam. This xe2x80x9cpattern bloomingxe2x80x9d can also result from the chemical interaction of scattered electrons. Backscattering from layers underlying the resist layer also can lead to unwanted exposure and pattern blooming.
The creation of precise patterns on layer 102a is facilitated by minimal spreading of the e-beam on passage through the resist, which favors the use of higher energy beams. Higher energy beams also produce unwanted backscatter from deeper layers of the substrate, but the backscatter is more diffuse and tends to lower contrast rather than diffuse edges. Use of high energy beams requires both higher voltage and higher beam currents. Low energy e-beams deposit a reasonably large fraction of the beam energy in the resist layer where it is needed to expose the resist. Therefore, low energy e-beams require less incident beam intensity (beam current) since more efficient use is made of the available beam intensity in exposing the resist. The energy deposited in the target is thus typically significantly less for low energy e-beams since both current and voltage are reduced from that used in high energy e-beam lithography. That is, energy deposited is the product of beam voltage, beam current and exposure duration (dwell time). When the beam voltage is increased the electrons leave less energy in the resist, so the current must also be increased to compensate to produce the correct degree of exposure.
By way of illustration and not limitation, we compare the exposure of resist by a 10 keV beam with that caused by a 50 keV beam. It is noticed experimentally that as beam energy increases, the current must increase almost linearly with beam energy to continue to expose the resist adequately. Thus, increasing the beam energy by a factor of 5 from 10 to 50 keV requires a concomitant increase in current by a factor of approximately 5 to adequately expose the resist. The energy deposited per e-beam pulse (or flash) is (volts)xc2x7(amps)xc2x7(pulse duration) which increases by a factor of approximately 25 in this example.
FIG. 1B depicts schematically in cross section (not to scale), e-beam 104b, incident on resist 103b, at high incident beam energies, typically around 50 keV. Beam spreading depicted as 5 in FIG. 1A is typically negligible in the resist layer 103b for high energy beam impacts as depicted in FIG. 1B. Such high energy beams tend to pass through resist layer 102b, mask layer 103b, and proceed well into the glass substrate 100b, before substantial beam spreading occurs. A heated zone 106 is thereby created in substrate 100b as the e-beam comes to rest. Typically, for electrons of about 50 kV in glass, zone 106 will be approximately 20 xcexcm in diameter (xcexcm=micron=10xe2x88x926 meter) with its centroid about 10 xcexcm below the upper surface of glass substrate 100b and for small flashes or round spots, the heated zone will have rotational symmetry about the axis defined by the incident e-beam.
As noted above, high voltage e-beams will typically deposit much more energy in substrate 100b than will low voltage beams, such energy increasing by approximately the square of the beam energy (as the need for increased current must also be met). The energy per pulse may not be substantial, but millions or indeed hundreds of millions of pulses may contribute to the proximity heating. Therefore, substantial heating of the substrate 100b may occur with high energy electron beams.
Direct heating of the resist layer by the incident e-beam is significant, but is readily predictable from the applied dose and, therefore, can be compensated by calibration. However, proximity heating of the point at which writing is currently occurring is variable since it is affected by heat conduction from regions within the substrate where heat was deposited by numerous (typically millions) of earlier pulses. Thus, proximity heating depends on the pattern being written and the timing and ordering of past pulses. This proximity heating results in pattern blooming, which must be compensated for if high energy beams are used.
FIG. 2 depicts the effects of pattern blooming that will typically result from increasing resist sensitivity due to proximity heating. The desired pattern of exposed resist 107 is shown in top view (FIG. 2A) and side view (FIG. 2B). The pattern and process designer will plan for e-beam exposure such that exposure point 109 (for example) occurs at the desired pattern boundary. However, increased resist sensitivity may lead to full exposure of resist by less-than-expected e-beam dose. That is, the pattern edge moves to position 110, resulting in a broadened pattern depicted by 108.
Proximity heating has been the subject of several calculations and measurements. Ralph et. al. describe methods for computing proximity heating by numerical integration of diffusion equations in xe2x80x9cProceedings of the Symposium on Electron and Ion Beam Science and Technology, Tenth International Conferencexe2x80x9d, p. 219-2330 (1983). Babin et. al. also describe methods for the numerical simulation of proximity heating and the comparison of such calculations with measured values. SPIE, Vol. 3048, p. 368-373 (1997) and J. Vac Sci Technol. B Vol. 16, pp. 3241-3247 (1998). Additional calculations of proximity heating and comparison with measured values have been reported by Yasuda et. al. in and J. Vac Sci Technol. B Vol. 12, pp. 1362-1366 (1994).
Calculations of proximity heating are typically based upon a numerical solution of the appropriate diffusion (partial differential) equations. Heat sources may be represented by analytic approximations, or derived directly by numerical Monte Carlo simulation of the electrons penetration into targets, including resists, masks and substrates. However, prior methods have proven in practice to be too slow in comparison with the speed of e-beam writing to allow real-time computation of proximity heating and adjustment of the writing process in response. More particularly, accurate predictions require using Monte Carlo simulations to represent single flash heat sources in the substrate and may use various techniques for solving the thermal diffusion equation including finite element, finite difference or elaborate analytical approximations. However, these methods tend to require lengthy computations and cannot be used directly in real time proximity heating correction.
As such, there is a need for an improved system and method for compensating for medium range substrate heating. There is a further need for compensating for medium range substrate heating in real time.
These and other drawbacks in the prior art are overcome in large part by a system and method according to embodiments of the present invention. More particularly, methods and procedures for determining resist temperature during exposure are described. The change of sensitivity of the resist with temperature is determined a priori experimentally. Once the temperature at the exposure point is known, as determined by techniques according to embodiments of the present invention, the exposure parameters can be changed using known techniques to compensate for the temperature change. Typically, the resist temperature rise predicted by embodiments of the present invention for the point of writing will be multiplied by a factor relating the temperature sensitivity of the resist. The result is a correction applied to the beam dose by controlling the beam current, or equivalently the flash dwell time, to provide more accurate resist exposure. Pattern blooming is thereby reduced.
Embodiments of the present invention relate to methods of predicting proximity heating in real-time as the writing proceeds, enabling dose compensation to be performed in real-time. Methods of achieving high-processing efficiency are described which allow the calculations to be done in real time as writing proceeds. An analytic shifted impulse response function is shown to give raster scan proximity heating results accurate to within a few percent. It is used for fast evaluation and design of correction schemes.
Methods according to implementations of the present invention prevent or mitigate pattern blooming as incident electrons heat the resist and broaden the region of exposure. Proximity heating is predicted in a very rapid manner, thereby making it possible to compute proximity heating corrections to the e-beam process in real-time as the process proceeds. The prediction occurs on a time scale comparable with the e-beam writing speeds. This real-time prediction of proximity heating allows the properties of the e-beam and/or the writing process to be adjusted while writing is underway to compensate for proximity heating. The resist temperature at the point of writing from the earlier written pattern is determined and the data enabling appropriate beam adjustments to compensate for the resulting exposure dose error are provided.